Electrodeposition coating process

ABSTRACT

Disclosed is an electrodeposition coating process which comprises: a first electrodeposition step in which an electrodeposition coated film is partially formed on a substrate surface to be coated with using a lead-free cationic electrodeposition coating composition which is modified so that a coated film deposited on the substrate surface from the coating composition (deposited film) has relatively low glass transition temperature; a second electrodeposition step in which an electrodeposition coated film is formed on an uncoated portion of the substrate surface with using a lead-free cationic electrodeposition coating composition which is modified so that the deposited film has relatively high glass transition temperature; and a step of baking the electrodeposition coated film to cure. The electrodeposition coating process makes difference of film thickness between the interior surface and the exterior surface to be small, makes smoothness of exterior surface to be excellent even if a small amount of coating composition is employed, and exert a little influence on the environment due to its low VOC, and low metal ion content.

FIELD OF THE INVENTION

[0001] The present invention relates to an electrodeposition coating process, specifically an electrodeposition coating process with using a lead-free cationic electrodeposition coating composition having low volatile organic content, low metal ion content.

BACKGROUND IF THE INVENTION

[0002] According to an electrodeposition coating method, a coated film can be formed even on narrow portions of a substrate to be coated having intricate shape, automatically and continuously. Therefore, the electrodeposition coating method is widely used for primer-coating a substrate having intricate shape and being required to have high rust resistance, such as an automobile body.

[0003] Further, an electrodeposition coating method is superior in utilization efficiency of a coating composition to the other coating method, and it has conventionally been conducted as an industrial coating method due to its economical advantage. The cationic electrodeposition coating method is conducted by dipping a substrate to be coated in an cationic electrodeposition coating composition, in which a voltage is applied with using the substrate as a cathode.

[0004] In order to improve corrosion resistance of an electrodeposition coated film, various metal catalysts including lead which act as an anti-corrosion agent have been added to the electrodeposition coating composition. However, it has been required in these days to cut down content of the metal catalyst employed in an electrodeposition coating composition because metal ion, specifically lead ion exerts a harmful effect on the environment.

[0005] On the other hand, in proportion as a concern for environmental problems has been grown, harmful air pollutants (HAPs) has been regulated in quantity more tightly over developed countries. An electrodeposition coating composition contains a volatile organic solvent to some extent as a solvent for synthesizing a resin, a flow aid for an electrodeposition coated film, a conditioning agent for coating operation, and the like. Therefore, an electrodeposition coating composition which contains HAPs in a substantial amount, may hardly be used if the environmental regulation is intensified.

[0006] It is also desired that consumption of a coating composition itself is reduced so as to make an influence on the environment more small.

[0007] Deposition of a coating solid which occurs in the course of electrodeposition coating is due to an electrochemical reaction. A coated film is deposited on a surface of a substrate to be coated by a voltage being applied to an electrodeposition coating composition. The substrate is electrically insulated when a coated film is deposited thereon, and electric resistance becomes large as the deposited film becomes thick.

[0008] As a result, deposition decreases at the portion on which a coated film has been formed. Alternatively deposition increases at the portion on which no coated film has been formed. Thus, a coated film sticks to an uncoated portion of the substrate, thereby coating process is completed. As described above, a coated film is sequentially formed on the uncoated portion of the substrate during the electrodeposition coating process. Such deposition property of the electrodeposition coating composition is referred to as “throwing power” throughout the specification. An electrodeposition coating composition having good throwing power can form a coated film which has even thickness over a coated surface.

[0009] Theoretically speaking, an insulative coated film is formed on a coated surface of the substrate sequentially on the electrodeposition coating process. Therefore, throwing power must be infinity and a coated film be made uniformly over the coated surface. However in fact, since an uncoated portion of the substrate is weak in voltage to be applied, the coating solid hardly sticks to that portion. Therefore, throwing power of an electrodeposition coating composition has not been sufficient, and unevenness of film thickness may have been occurred.

[0010] An electrodeposition coated film is usually employed as a primer coating which aims at preventing corrosion or rust from generating on a substrate to be coated. Therefore, even if the substrate is complex in structure, the coated film must have not less than a certain value of thickness at the whole portion. Thus, when unevenness of film thickness is present, the thicker portion is being overcoated, and it means that the coating composition has been consumed excessively.

[0011] Therefore, in order to increase utilization efficiency of a coating composition, throwing power have to be improved, in addition, difference of film thickness between the electrodeposition coated film formed on an interior surface of the substrate and that formed on an exterior surface, have to be made small.

[0012] On the other hand, reducing consumption of a coating composition means that deposited film is made small in film thickness, as a result smoothness of the deposited film becomes poor. This has been a substantial problem so far as an automobile coating use for which excellent appearance is required.

SUMMARY OF THE INVENTION

[0013] The present invention solves the above-mentioned problems of the background art, and it is an object of the present invention to provide an electrodeposition coating process which makes difference of film thickness between the interior surface and the exterior surface to be small, makes smoothness of exterior surface to be excellent even if a small amount of coating composition is employed, and exert a little influence on the environment due to its low VOC, and low metal ion content.

[0014] The present invention provides an electrodeposition coating process comprising:

[0015] a first electrodeposition step in which an electrodeposition coated film is partially formed on a substrate surface to be coated with using a lead-free cationic electrodeposition coating composition which comprises an aqueous medium, a binder resin composed of a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in the aqueous medium, a neutralizing acid in order to neutralize the cationic epoxy resin, an organic solvent, and a metal catalyst, and which has a volatile organic content of 1% by weight or less, a metal ion content of 500 ppm or less and a neutralizing acid amount of 10 to 30 mg equivalent based on 100 g of binder resin solid content, and an electrodeposition coated film deposited on the substrate surface from the coating composition has a glass transition temperature of not more than 0° C.;

[0016] a second electrodeposition step in which an electrodeposition coated film is formed on an uncoated portion of the substrate surface with using a lead-free cationic electrodeposition coating composition which comprises an aqueous medium, a binder resin composed of a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in the aqueous medium, a neutralizing acid in order to neutralize the cationic epoxy resin, an organic solvent, and a metal catalyst, and which has a volatile organic content of 1% by weight or less, a metal ion content of 500 ppm or less and a neutralizing acid amount of 10 to 30 mg equivalent based on 100 g of binder resin solid content, and an electrodeposition coated film deposited on the substrate surface from the coating composition has a glass transition temperature of 5 to 20° C.; and

[0017] a step of baking the electrodeposition coated film to cure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a perspective view which shows an example of box employed for evaluating the throwing power.

[0019]FIG. 2 is a cross sectional view which schematically shows one embodiment of the process for evaluating the throwing power.

DETAILED DESCRIPTION OF THE INVENTION

[0020] An electrodeposition coating composition contains binder, pigment, solvent and various kinds of additives such as anticorrosion agent in an aqueous medium. The binder includes a cationic resin having a functional group and a curing agent for curing the cationic resin. As the aqueous medium, ion-exchanged water, deionized water, and the like are employed.

[0021] The wording “lead-free” means that lead is not substantially contained, i.e., lead is not present in an amount so as to exert an influence on the environment. Specifically it means that lead is not present in an electrodeposition bath beyond 50 ppm, preferably beyond 20 ppm.

[0022] In the present invention, a cationic epoxy resin which is obtainable by allowing an active hydrogen compound such as amine to react with an epoxy ring of an epoxy resin to introduce a cationic group by opening the epoxy group, is used as the cationic resin, and a block polyisocyanate in which an isocyanate group of polyisocyanate is blocked is used as the curing agent.

[0023] Cationic epoxy resin

[0024] The cationic epoxy resin used in the present invention includes an amine modified epoxy resin. The cationic epoxy resin may be those disclosed in Japanese Patent Kokai Publications No. Sho 54-4978 and Sho 56-34186.

[0025] The cationic epoxy resin is typically prepared by opening all epoxy rings in a bisphenol type epoxy resin by an active hydrogen compound which can introduce a cationic group, or by opening a part of epoxy rings by the other active hydrogen compound, while opening the remaining epoxy rings by an active hydrogen compound which can introduce a cationic group.

[0026] A typical example of the bisphenol type epoxy resin is the bisphenol A type or the bisphenol F type epoxy resin. The former is commercially available in the names of EPICOAT™ 828 (Yuka-Shell Epoxy Co. Ltd., epoxy equivalent 180 to 190), EPICOAT™ 1001 (epoxy equivalent 450 to 500), EPICOAT™ 1010 (epoxy equivalent 3000 to 4000) and the like, and the latter is commercially available in the name of EPICOAT™ 807 (epoxy equivalent 170) and the like.

[0027] An oxazolidone ring containing epoxy resin as described by chemical formula 3 of paragraph [0004] in Japanese Patent Kokai Publication No. Hei 5-306327 may be used as the cationic epoxy resin. This is because a coated film which is superior in throwing power, heat resistance and corrosion resistance can be obtained.

[0028] An oxazolidone ring is introduced into an epoxy resin, for example, by the step of heating a block polyisocyanate which is blocked by lower alcohol such as methanol and a polyepoxide in the presence of basic catalyst with removing lower alcohol generated as byproduct by distillation.

[0029] Especially preferred epoxy resin is an oxazolidone ring containing epoxy resin. This is because a coated film which is superior in heat resistance and corrosion resistance, as well as superior in shock resistance can be obtained.

[0030] It is known that an oxazolidone ring containing epoxy resin can be obtained by allowing a bi-functional epoxy resin to react with a diisocyanate that is blocked by monoalcohol (i.e., bisurethane). Specific examples and preparation methods of the oxazolidone ring containing epoxy resin are disclosed, for example, in paragraphs [0012] to [0047] of Japanese Patent Kokai Publication No. 2000-128959.

[0031] Block polyisocyanate curing agent

[0032] Polyisocyanate used for the curing agent of the present invention refers to a compound having two or more isocyanate groups in one molecule. For example, as the polyisocyanate, it may be any of aliphatic, alicyclic, aromatic and aromatic-aliphatic.

[0033] Specific examples of the polyisocyanate include aromatic diisocyanates such as tolylenediisocyanate (TDI), diphenylmethanediisocyanate (MDI), p-phenylenediisocyanate and naphthalenediisocyanate; aliphatic diisocyanates having 3 to 12 carbon atoms such as hexamethylenediisocyanate (HDI), 2,2,4-trimethylhexanediisocyanate and lysinediisocyanate; alicyclic diisocyanates having 5 to 18 carbon atoms such as 1,4-cyclohexanediisocyanate (CDI), isophoronediisocyanate (IPDI), 4,4′-dicyclohexylmethanediisocyanate (hydrogenated MDI), methylcyclohexanediisocyanate, isopropylidene dicyclohexyl-4,4′-diisocyanate and 1,3-isocyanatomethyl cyclohexane (hydrogenated XDI), hydrogenated TDI, 2,5- or 2,6-bis(isocyanatometyl) bicyclo [2.2.1] heptane (also referred to as norbornanediisocyanate); aliphatic diisocyanates having an aromatic ring such as xylylenediisocyanate (XDI) and tetramethylxylylenediisocyanate (TMXDI); and modified diisocyanates (urethanation compounds, carbodiimide, urethodione, urethoimine, biuret and/or isocyanurate modified compounds). These may be used alone or in combination of two or more.

[0034] An adduct or a prepolymer that can be obtained by reacting polyisocyanate with polyalcohol such as ethylene glycol, propylene glycol, trimethylolpropane or hexatriol at a NCO/OH ratio of not less than 2 can also be used as a curing agent.

[0035] A block agent is those capable of adding to a polyisocyanate group, and reproducing a free isocyanate when heated to dissociation temperature though it is stable at ambient temperature.

[0036] As a block agent, those conventionally employed such as ε-caprolactam and ethylene glycol monobutyl ether may be employed. However, many of the volatile block agents among these are regulated as being HAPs, and preferably be used in minimum amount.

[0037] Pigment

[0038] An electrodeposition coating composition generally contains pigment as a colorant. Examples of such pigment include titanium white, carbon black and colcothar. However, it is preferred that an electrodeposition coating composition employed in the present invention does not contain pigment. This is because throwing power of the coating composition is improved.

[0039] As to an extender pigment, or a rust preventive pigment, they may be included in order to provide corrosion resistance to a coated film. The amount however is preferably a ratio of 1/9 or less by weight based on a resin solid contained in the coating composition (PV). If the ratio of the pigment is more than 1/9 by weight, throwing power of the coating composition becomes poor, and it results in wasteful consumption of the coating composition.

[0040] Examples of such pigment may be employed in the lead-free cationic electrodeposition coating composition employed in the present invention include extender pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica, clay and silica, rust preventive pigments such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripoliphosphate, zinc molybdate, aluminum molybdate, calcium molybdate, aluminum phosphomolybdate, and aluminum zinc phosphomolybdate.

[0041] Pigment dispersion paste

[0042] When pigment is used as a component of an electrodeposition coating composition, generally, the pigment is dispersed in an aqueous medium at high concentration in advance and made into a paste form. This is because pigment is of the powder form, and it is difficult to be dispersed uniformly into low concentration which is used in the electrodeposition coating composition, by one step process. Such a paste is generally referred to as a pigment dispersion paste.

[0043] A pigment dispersion paste is prepared by allowing pigment to disperse in an aqueous medium together with a pigment dispersing resin. Generally, as the pigment dispersing resin, cationic or nonionic low molecular weight surface active agents or cationic polymers such as modified epoxy resins having a quaternary ammonium group and/or a tertiary sulfonium group are used. As the aqueous medium, ion-exchange water or water containing a small amount of alcohol is used. Generally, the pigment dispersing resin and the pigment are used in a solid content ratio of 5 to 40 parts by weight to 20 to 50 parts by weight.

[0044] Metal catalyst

[0045] A metal catalyst may be included in the lead-free cationic electrodeposition coating composition employed in the present invention in the form of metal ion as a catalyst for improving corrosion resistance of a coated film. The metal ion includes preferably cerium ion, bismuth ion, copper ion, and zinc ion. These are incorporated in the electrodeposition coating composition in the form of an eluted component derived from salts combined with suitable acids, or pigments composed of the corresponding metal. The acids may be any of inorganic or organic acids described later as a neutralizing acid such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid. Preferred acid is the acetic acid.

[0046] The lead-free cationic electrodeposition coating composition employed in the present invention contains the metal catalyst in an amount so that metal ion concentration in the coating composition is 500 ppm or less. This is because an influence exerted on the environment is minimized. Preferably, the metal ion concentration in the coating composition is 200 to 400 ppm.

[0047] As to an amount of the metal ion, when the pigment is employed in the coating composition, it must be noticed that the metal ion may also be eluted from the pigment. Thus, the combination amount of the metal catalyst should be controlled with considering an amount of the metal ion eluted from the pigment. Examples of the metal ion eluted from the pigment include zinc ion, molybdenum ion, aluminum ion and the like.

[0048] If the metal ion is included in the electrodeposition coating composition in an amount of more than 500 ppm, an influence exerted on the environment becomes too large, deposition property of a binder resin becomes poor, and throwing power of the coating composition becomes poor. The metal ion concentration of the electrodeposition coating composition is measured by conducting atomic absorption analysis on a supernatant liquid obtained by centrifugal separation of the coating composition.

[0049] Lead-free electrodeposition coating composition

[0050] A cationic electrodeposition coating composition employed in the present invention is prepared by dispersing the metal catalyst, the cationic epoxy resin, the block polyisocyanate curing agent, and the pigment dispersion paste in an aqueous medium. In addition to these, the aqueous medium usually includes a neutralizing acid so that the cationic epoxy resin is neutralized to improve dispersibility of a binder resin emulsion. The neutralizing acid includes inorganic and organic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid.

[0051] When the coating composition includes a large amount of neutralizing acid, neutralize ratio of the cationic epoxy resin becomes high, the binder resin particles have high affinity with the aqueous medium, and dispersion stability thereof increases. This means that the binder resin particles hardly deposit on the substrate when electrodeposition coating is conducted, and means poor deposition property.

[0052] On the other hand, when the coating composition includes a small amount of neutralizing agent, neutralize ratio of the cationic epoxy resin becomes low, the binder resin particles have low affinity with the aqueous medium, and dispersion stability thereof decreases. This means that the binder resin particles easily deposit on the substrate when electrodeposition coating is conducted, and means good deposition property.

[0053] Thus, in order to improve throwing power of the electrodeposition coating composition, it is preferred that an amount of the neutralizing acid included in the coating composition is reduced to control neutralize ratio of the cationic epoxy resin to low level.

[0054] The neutralizing acid is specifically contained in an amount so as to be 10 to 30 mg eq., preferably 15 to 25 mg eq. based on 100 g of a resin solid of the binder which includes the cationic epoxy resin and the block isocyanate curing agent. If the amount of the neutralizing agent is less than 10 mg eq., the binder resin particles are insufficient or lack in affinity with water, and poor in dispersion stability. If the amount is more than 30 mg eq., deposition property of the coating solid decreases, a large quantity of electricity is required for conducting deposition, and throwing power also becomes poor.

[0055] In the present specification, the amount of the neutralizing acid is represented by milligram equivalent value based on 100 g of the binder resin solid which is contained in the coating composition, and is referred to as MEQ(A).

[0056] The amount of the block polyisocyanate curing agent is such that it is satisfactory to react with an active hydrogen containing functional group such as a primary, secondary and/or tertiary amino group or a hydroxyl group in the cationic epoxy resin at the time of heat curing and to give a preferable cured coated film. It is generally 50/50 to 90/10, preferably 65/35 to 80/20 when represented by solid content ratio by weight of the cationic epoxy resin based on the block polyisocyanate curing agent.

[0057] The cationic electrodeposition coating composition employed in the present invention may contain a tin compound such as dibutyltin dilaurate or dibutyltin oxide, or a usual urethane cleavage catalyst. The addition amount thereof is preferably 0.1 to 5.0% by weight of a resin solid.

[0058] An organic solvent is essentially required as a solvent when resin components such as a cationic epoxy resin, a block polyisocyanate curing agent, and a pigment dispersing resin and the like are prepared, and complicated procedure is required for removing the organic solvent completely. Further, when an organic solvent is contained in a binder resin, fluidity of coated film at the time of film forming is improved, and smoothness of the coated film is improved.

[0059] Examples of the organic solvent usually contained in the coating composition include ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylhexyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, propylene glycol monophenyl ether, and the like.

[0060] Therefore, an organic solvent have not been completely removed from a resin component conventionally, on the contrary an organic solvent is further added to the electrodeposition coating composition, thereby VOC (volatile organic content) of the coating composition is adjusted about from 1 to 5% by weight. In this context, the “volatile organic” means the organic solvent having a boiling point of 250° C. or less. The examples include the above described organic solvents.

[0061] On the other hand, the lead-free cationic electrodeposition coating composition employed in the present invention has the organic solvent content lower than that used to be. This is because a bad influence on the environment is prevented. Specifically, the coating composition is controlled to have a VOC of not more than 1% by weight, preferably 0.5 to 0.8% by weight, more preferably 0.2 to 0.5% by weight. If VOC of the coating composition is more than 1% by weight, an influence exerted on the environment becomes large, electric resistance of the coated film decreases due to flowability improvement of the coated film, and throwing power becomes poor.

[0062] As to the method for controlling VOC not more than 1% by weight, for example, an organic solvent employed for viscosity control at the time of conducting reaction may be reduced in its content by the reaction being conducted at higher temperature in lower solvent. An organic solvent inevitably employed at the time of conducting reaction, may be recovered by a desolvation process by such a means of employing a low boiling-point solvent, thereby VOC of the end product may be reduced. An organic solvent employed for viscosity control at the time of coating may be reduced in its content by modifying the resin with soft segment so as to have lower viscosity.

[0063] VOC may be determined by measuring amount of an organic solvent contained in the electrodeposition coating composition according to the gas liquid chromatography method by using internal standard.

[0064] In addition, the lead-free cationic electrodeposition coating composition employed in the present invention may contain commonly used additives for coating composition such as water miscible organic solvent, surface active agent, oxidation inhibiting agent and ultraviolet absorbing agent.

[0065] Electrodeposition coating process

[0066] An electrodeposition coating process of the present invention is conducted with using the lead-free cationic electrodeposition coating composition. In this instance, an electrodeposition coated film is formed on a substrate surface to be coated by conducting a first electrodeposition step and a second electrodeposition step. The first electrodeposition step is conducted with using the electrodeposition coating composition modified so that a coated film deposited on the substrate surface therefrom (deposited film) has relatively low Tg (a first electrodeposition coating composition). The second electrodeposition step is conducted with using the electrodeposition coating composition modified so that the deposited film has relatively high Tg (a second electrodeposition coating composition).

[0067] The Tg of the binder resin described herein means the theoretical Tg value which may be work out from Tg values of the respective component resins. The Tg value may be calculated according to the Fox equation as shown below:

1/Tg=w ₁ /Tg ₁ +w ₂ /Tg ₂ + . . . +w _(n) /Tg _(n)

[0068] wherein w_(n) represents percent by weight of the n-th resin component, and Tg_(n) represents glass transition temperature of the n-th resin component (provided temperature unit is Kelvin).

[0069] As to Tg of the every resins, the value respectively measured by detecting thermo-alternation accompanied with glass transition of the resin with using a differential scanning calorimeter, was employed.

[0070] The first electrodeposition coating composition and the second electrodeposition coating composition are substantially the same in volatile organic content, metal ion content, and neutralizing acid amount, but different in Tg of the deposited film. That is, the deposited film obtained in the first electrodeposition step from the first electrodeposition coating composition has a Tg of not more than 0° C., preferably −20 to 0° C., more preferably −10 to 0° C. The deposited film obtained in the second electrodeposition step from the second electrodeposition coating composition has a Tg of 5 to 20° C., preferably 5 to 15° C. The Tg of the deposited film may easily be varied by those skilled in the art with arranging composition of the binder resins contained in the electrodeposition coating composition.

[0071] If Tg of the deposited film obtained in the first electrodeposition step is more than 0° C., smoothness of less than 20 um thick becomes poor. If Tg of the deposited film obtained in the second electrodeposition step is less than 5° C., throwing power becomes insufficient, therefore the interior deposited film may become less than 10 um thick, results in poor corrosion resistance. If the Tg is more than 20° C., the deposited film does not sufficiently flow by heat, it becomes difficult to form film resistance which is required for good throwing power, results in poor throwing power.

[0072] A substrate to be coated in the electrodeposition coating process, is not limited to but those having conductivity, and iron plate, steel plate, aluminum plate, and surface-treated objects thereof, and molded objects thereof can be exemplified.

[0073] Electrodeposition coating is carried out, in general, by filling an electrodeposition bath with the electrodeposition coating composition, and applying a voltage of usually 50 to 450 V between the substrate serving as cathode and anode. If the applied voltage is less than 50 V, the electrodeposition becomes insufficient, and if the applied voltage exceeds 450 V, power consumption increases, which leads lack of economy. Temperature of the electrodeposition bath in the case of applying the voltage is, generally 10 to 45° C.

[0074] The electrodeposition process preferably comprises the steps of (i) immersing a substrate to be coated in an electrodeposition coating composition, and (ii) applying a voltage between the substrate as cathode and anode to cause deposition of coated film. Also, the period of time for applying the voltage can be generally 2 to 4 minutes, though it varies with the electrodeposition condition. The electrodeposition bath temperature is usually controlled at 10 to 45° C.

[0075] The first electrodeposition step is conducted so that the deposited film obtained has a maximum film thickness of 8 to 20 um, preferably 10 to 15 um. If the maximum film thickness is less than 8 um, surface smoothness becomes poor, and if the maximum film thickness is more than 20 um, it results in wasteful consumption.

[0076] The second electrodeposition step is conducted so that the deposited film obtained has a maximum film thickness of 8 to 15 um, preferably 10 to 15 um. If the maximum film thickness is less than 8 um, corrosion resistance becomes poor, and if the maximum film thickness is more than 15 um, it also results in wasteful consumption.

[0077] According to the electrodeposition coating process, a coated film is mainly formed on an exterior surface portion of the substrate in the first electrodeposition step, and a coated film is mainly formed on an interior surface portion of the substrate in the second electrodeposition step. As a result, difference of film thickness between the interior surface and the exterior surface becomes little or nothing. In this context, the exterior surface portion means a portion which is visible from the outside of a substrate to be coated. The interior surface portion means an inside portion of a bursiform substrate to be coated.

[0078] The electrodeposition coated film obtained in the manner as described above is baked at a temperature of 120 to 260° C., preferably 160 to 220° C. for 10 to 30 minutes to be cured directly or after being washed with water after completion of the electrodeposition process.

[0079] The present invention will be further explained in detail in accordance with the following examples, however, the present invention is not limited to these examples. In the examples, “part” and “%” are based on weight unless otherwise specified. “Epoxy equivalent” and “amine equivalent” are values per solid content.

EXAMPLES Preparation Example 1

[0080] Preparation of amine modified epoxy resin

[0081] 92 parts of 2,4-/2,6-tolylenediisocyanate (weight ratio=8/2), 95 parts of methyl isobutyl ketone (hereinafter, referred to as MIBK) and 0.5 part of dibutyltin dilaurate were loaded to a flask equipped with a stirrer, a cooling tube, a nitrogen introducing tube, a thermometer and a dropping funnel. 21 parts of methanol was added while stirring the mixture.

[0082] Starting at room temperature, the reaction mixture was allowed to rise to 60° C. by exothermic, the reaction was retained for 30 minutes, and 50 parts of ethylene glycol mono-2-ethylhexyl ether was dropped from the dropping funnel. Furthermore, 53 parts of bisphenol A-propylene oxide 5 mol adduct was added. The reaction was carried out mainly in the temperature range of 60 to 65° C., and continued until absorption based on an isocyanate group disappeared in IR spectrum measurement.

[0083] Next, 365 parts of bisphenol A type epoxy resin of epoxy equivalent 188 synthesized from bisphenol A and epichlorohydrin in accordance with a known method was added to the reaction mixture and heated to 125° C. After that, 1.0 part of benzyldimetylamine was added and allowed to react at 130° C. until epoxy equivalent became 410.

[0084] Subsequently, 61 parts of bisphenol A and 33 parts of octylic acid was added and allowed to react at 120° C. to achieve epoxy equivalent of 1190. Thereafter, the reaction mixture was cooled, and 11 parts of diethanolamine, 24 parts of N-ethylethanolamine and 25 parts of 79% solution in MIBK of ketimined aminoethyl ethanolamine were added, and was allowed to react for 2 hours at 110° C. Then, the reaction mixture was diluted with MIBK until NV solid became 80%, and an amine modified epoxy resin which has a glass transition temperature (Tg) of 2° C. (solid content: 80%) was obtained.

Preparation Example 2

[0085] Preparation of amine modified epoxy resin

[0086] 92 parts of 2,4-/2,6-tolylenediisocyanate (weight ratio=8/2), 95 parts of methyl isobutyl ketone (hereinafter, referred to as MIBK) and 0.5 part of dibutyltin dilaurate were loaded to a flask equipped with a stirrer, a cooling tube, a nitrogen introducing tube, a thermometer and a dropping funnel. 21 parts of methanol was added while stirring the mixture.

[0087] Starting at room temperature, the reaction mixture was allowed to rise to 60° C. by exothermic, the reaction was retained for 30 minutes, and 57 parts of ethylene glycol mono-2-ethylhexyl ether was dropped from the dropping funnel. Furthermore, 42 parts of bisphenol A-propylene oxide 5 mol adduct was added. The reaction was carried out mainly in the temperature range of 60 to 65° C., and continued until absorption based on an isocyanate group disappeared in IR spectrum measurement.

[0088] Next, 365 parts of bisphenol A type epoxy resin of epoxy equivalent 188 synthesized from bisphenol A and epichlorohydrin in accordance with a known method was added to the reaction mixture and heated to 125° C. After that, 1.0 part of benzyldimetylamine was added and allowed to react at 130° C. until epoxy equivalent became 410.

[0089] Subsequently, 87 parts of bisphenol A was added and allowed to react at 120° C. to achieve epoxy equivalent of 1190. Thereafter, the reaction mixture was cooled, and 11 parts of diethanolamine, 24 parts of N-ethylethanolamine and 25 parts of 79% solution in MIBK of ketimined aminoethyl ethanolamine were added, and was allowed to react for 2 hours at 110° C. Then, the reaction mixture was diluted with MIBK until NV solid became 80%, and an amine modified epoxy resin which has a Tg of 22° C. (solid content: 80%) was obtained.

Preparation Example 3

[0090] Preparation of block polyisocyanate curing agent

[0091] 1250 parts of diphenylmethanediisocyanate, 266.4 parts of MIBK were loaded to a flask, this was heated to 80° C., and 2.5 parts of dibutyltin dilaurate were added to this. A solution of 226 parts of ε-caprolactam dissolved in 944 parts of ethylene glycol monobutyl ether was dropped thereto at 80° C. over 2 hours. The reaction was retained at 100° C. for 4 hours, it was confirmed that absorption based on an isocyanate group disappeared in IR spectrum measurement, and left to be cooled. 336.1 parts of MIBK were added and thereby, a block polyisocyanate curing agent having a Tg of 0° C. was obtained.

Preparation Example 4

[0092] Preparation of pigment dispersing resin

[0093] 222.0 parts of isophoronediisocyanate (hereinafter, referred to as IPDI) was loaded in a reaction vessel equipped with a stirrer, a cooling tube, a nitrogen introducing tube and a thermometer, and after diluted with 39.1 parts of MIBK, 0.2 part of dibutyltin dilaurate was added. Then, the reaction mixture was heated to 50° C., and 131.5 parts of 2-ethyl hexanol was dropped under dry nitrogen atmosphere over 2 hours with stirring. Reaction temperature was kept at 50° C. by cooling as necessary. As a result of this, 2-ethyl hexanol half blocked IPDI (solid content: 90%) was obtained.

[0094] 87.2 parts of dimethylethanolamine, 117.6 parts of 75% aqueous solution of lactic acid, and 39.2 parts of ethylene glycol monobutyl ether were added to a suitable reaction vessel, and the reaction mixture was stirred at 65° C. for half an hour to prepare a quaternarizing agent.

[0095] Subsequently 710.0 parts of EPON 829 (bisphenol A type epoxy resin manufactured by Shell Chemical Company, epoxy equivalents 193 to 203), and 289.6 parts of bisphenol A were loaded to a reaction vessel. The reaction mixture was heated to 150 to 160° C. under nitrogen atmosphere, exothermic reaction was initially occurred. Heating was continued at 150 to 160° C. for about 1 hour, the reaction mixture was then cooled to 120° C., 498.8 parts of the prepared 2-ethyl hexanol half-blocked IPDI (MIBK solution) was added.

[0096] The reaction mixture was held at 110 to 120° C. for 1 hour, 463.4 parts of ethylene glycol monobutyl ether were added, the mixture was cooled to 85 to 95° C., homogenized, and 196.7 parts of the prepared quaternarizing agent was added thereto. The reaction mixture was held at 85 to 95° C. until the acid value became 1, 964 parts of deionized water were added to finalize quaternarization of an epoxy-bisphenol A resin and to obtain a pigment dispersing resin having quaternary ammonium moiety (Tg: 5° C., solid content: 50%).

Preparation Example 5

[0097] Preparation of pigment dispersion paste

[0098] 120 parts of the pigment dispersing resin obtained in Preparation example 4, 2.0 parts of carbon black, 100.0 parts of kaolin, 80.0 parts of titanium dioxide, 18.0 parts of aluminum phosphomolibudate and 221.7 parts of ion-exchange water were loaded into a sand grinding mill, and they were dispersed until grain size was not more than 10 um, to obtain a pigment dispersion paste (solid content: 48%).

Preparation Example 6

[0099] Preparation of first electrodeposition coating composition

[0100] The amine modified epoxy resin obtained in Preparation example 1 and the block polyisocyanate curing agent obtained in Preparation example 3 were uniformly mixed in solid content ratio of 70:30. Bisphenol A-propylene oxide 6 mol adduct (Tg: −40° C.) was added so that the amount based on solid content was 10%, and ethylene glycol 2-ethylhexyl ether were of methyl isobutyl ketone (hereinafter, referred to as MIBK) and 0.5 part of acetic acid was added to this so that milligram equivalent value of acid based on 100 g of the binder resin solid content MEQ(A) was 30, and ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain an emulsion having a solid content of 36%.

[0101] 1500 parts of this emulsion, 540 parts of the pigment dispersion paste obtained in Preparation example 5, 1920 parts of ion-exchanged water, 40 parts of 10% cerium acetate aqueous solution, and 10 parts of dibutyltin oxide were mixed, and a cationic electrodeposition coating composition having a solid content of 20.0% was obtained. This electrodeposition coating composition had a volatile organic content in the coating composition (VOC) of 0.9%, a milligram equivalent value of acid based on 100 g of the binder resin solid content (MEQ(A)) of 24.7, and a total concentration of the eluted cerium ion and zinc ion of 380 ppm. Tg of the electrodeposition coated film deposited therefrom (uncured) was calculated from the Tgs of the respective component resins and found to be −3° C.

Preparation Example 7

[0102] Preparation of second electrodeposition coating composition

[0103] The amine modified epoxy resins obtained in Preparation examples 1 and 2, and the block polyisocyanate curing agent obtained in Preparation example 3 were uniformly mixed in solid content ratio of 20:50:30. Glacial acetic acid was added to this so that milligram equivalent value of acid based on 100 g of the binder resin solid content MEQ(A) was 20, and ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain an emulsion having a solid content of 36%.

[0104] 2220 parts of this emulsion, 1740 parts of ion-exchanged water, 40 parts of 10% cerium acetate aqueous solution, and 10 parts of dibutyltin oxide were mixed, and a cationic electrodeposition coating composition having a solid content of 20.0% was obtained. This electrodeposition coating composition had a VOC of 0.5%, a MEQ(A) of 25.2, a total concentration of the eluted cerium ion and zinc ion of 200 ppm. Tg of the electrodeposition coated film deposited therefrom (uncured) was calculated from the Tgs of the respective component resins and found to be 11° C.

Preparation Example 8

[0105] Preparation of second electrodeposition coating composition

[0106] The amine modified epoxy resin obtained in Preparation example 2 and the block polyisocyanate curing agent obtained in Preparation example 3 were uniformly mixed in solid content ratio of 70:30. Glacial acetic acid was added to this so that milligram equivalent value of acid based on 100 g of the binder resin solid content MEQ(A) was 25, and ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain an emulsion having a solid content of 36%.

[0107] 1500 parts of this emulsion, 540 parts of pigment dispersion paste obtained in Preparation example 5, 1940 parts of ion-exchanged water, 20 parts of 10% cerium acetate aqueous solution, and 10 parts of dibutyltin oxide were mixed, and a cationic electrodeposition coating composition having a solid content of 20.0% was obtained. This electrodeposition coating composition had a VOC of 0.5%, a MEQ(A) of 21.5, a total concentration of the eluted cerium ion and zinc ion of 205 ppm. Tg of the electrodeposition coated film deposited therefrom (uncured) was calculated from the Tgs of the respective component resins and found to be 14° C.

Example 1

[0108] As shown in FIG. 1, four steel plates 11 to 14 (JIS G 3141 SPCC-SD) treated with zinc phosphorate (SURFDINE SD-5000 available from Nippon Paint K. K.) were placed vertically in parallel at 20 mm interval, and lower parts of the both side planes and the bottom plane were covered by an insulating material such as an adhesive tape to prepare a box 10. The steel plates 11 to 13, except plate 14, had an opening 15 of 8 mm phi.

[0109] 4 litter of the first cationic electrodeposition coating composition obtained in Preparation example 6 was filled in a vinyl chloride vessel to obtain a first electrodeposition bath. As shown in FIG. 2, the box 10 was dipped in the electrodeposition vessel 20 filled with the electrodeposition coating composition 21. In this situation, the coating composition 21 came in and out the box 10 only through the openings 15.

[0110] The coating composition 21 was stirred by a magnetic stirrer (not shown). The steel plates 11 to 14 were electrically connected, and a counter electrode 22 was placed at the position of 150 mm distant from the nearest steel plate 11. A voltage was applied between the steel plates 11 to 14 used as a cathode, and the counter electrode 22 used as an anode, thereby the steel plates were electrodeposition coated. The coating step was conducted under a bath temperature of 30° C. and an applied voltage of 170 V, for 3 minutes.

[0111] The second cationic electrodeposition coating composition obtained in Preparation example 7 was filled in a vinyl chloride vessel to obtain a second electrodeposition bath. The box 10 was took out from the first electrodeposition bath, and dipped in the second electrodeposition bath as shown in FIG. 2 in the same manner as described above. The coating step was then conducted under a bath temperature of 30° C. and an applied voltage of 230 V, for 3 minutes.

[0112] The box 10 was then took out from the second electrodeposition bath, washed with water, and subjected to setting for 10 minutes. The box 10 was placed in a drying oven set to 170° C., heated for 25 minutes, and the coated film was cured. After the box 10 was cooled, thickness of the film on an anode facing surface A of the steel plate 11 nearest from the counter electrode 22, was measured as film thickness of a coated film formed on an exterior surface of the substrate. Then, thickness of the film on an anode facing surface G of the steel plate 14 farthest from the counter electrode 22, was measured as film thickness of a coated film formed on an interior surface of the substrate. As a result, the film formed on the exterior surface A of the substrate had a thickness of 15 um, and the film formed on the interior surface G had a thickness of 12 um.

[0113] Surface roughness (Ra) of the cured coated film on the surface A was measured by using a surface roughness meter SURFTEST-211 (manufactured by Mitsutoyo K.K.) under a cut off of 0.8 mm, and a scan length of 4 mm, and found to be 0.12 um. The smaller Ra value means the smoother surface condition.

Example 2

[0114] A cured coated film was obtained according to substantially the same manner as described in Example 1, except that the second cationic electrodeposition coating composition obtained in Preparation example 8 was employed. The film formed on the exterior surface A of the substrate had a thickness of 13 um, and the film formed on the interior surface G had a thickness of 11 um. The Ra value of the cured coated film on the surface A was measured to be 0.15 um.

Comparative Example 1

[0115] The amine modified epoxy resin obtained in Preparation example 1 and the block polyisocyanate curing agent obtained in Preparation example 3 were uniformly mixed in solid content ratio of 70:30. Ethylene glycol 2-ethylhexyl ether was then added so that the amount based on solid content was 5%, and glacial acetic acid was added so that milligram equivalent value of acid based on 100 g of the binder resin solid content MEQ(A) was 35, and ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain an emulsion having a solid content of 36%.

[0116] 1500 parts of this emulsion, 540 parts of the pigment dispersion paste obtained in Preparation example 6, 1900 parts of ion-exchanged water, 60 parts of 10% cerium acetate aqueous solution, and 10 parts of dibutyltin oxide were mixed, and a cationic electrodeposition coating composition having a solid content of 20.0% was obtained. This electrodeposition coating composition had a volatile organic content in the coating composition (VOC) of 1.1%, a milligram equivalent value of acid based on 100 g of the binder resin solid content (MEQ(A)) of 29.7, and a total concentration of the eluted cerium ion and zinc ion of 610 ppm. Tg of the electrodeposition coated film deposited therefrom (uncured) was calculated from the Tgs of the respective component resins and found to be 2° C.

[0117] As shown in FIG. 1, four steel plates 11 to 14 (JIS G 3141 SPCC-SD) treated with zinc phosphorate (SURFDINE SD-5000 available from Nippon Paint K.K.) were placed vertically in parallel at 20 mm interval, and lower parts of the both side planes and the bottom plane were covered by an insulating material such as an adhesive tape to prepare a box 10. The steel plates 11 to 13, except plate 14, had an opening 15 of 8 mm phi.

[0118] 4 litter of the cationic electrodeposition coating composition was filled in a vinyl chloride vessel to obtain an electrodeposition bath. As shown in FIG. 2, the box 10 was dipped in the electrodeposition vessel 20 filled with the electrodeposition coating composition 21.

[0119] The coating composition 21 was stirred by a magnetic stirrer (not shown). The steel plates 11 to 14 were electrically connected, and a counter electrode 22 was placed at the position of 150 mm distant from the nearest steel plate 11. A voltage was applied between the steel plates 11 to 14 used as a cathode, and the counter electrode 22 used as an anode, thereby the steel plates were electrodeposition coated. The coating step was conducted under a bath temperature of 30° C. and an applied voltage of 200 V, for 3 minutes.

[0120] The box 10 was then took out from the electrodeposition bath, washed with water, and subjected to setting for 10 minutes. The box 10 was placed in a drying oven set to 170° C., heated for 25 minutes, and the coated film was cured. After the box 10 was cooled, thickness of the film on an anode facing surface A of the steel plate 11 nearest from the counter electrode 22, was measured as film thickness of a coated film formed on an exterior surface of the substrate. Then, thickness of the film on an anode facing surface G of the steel plate 14 farthest from the counter electrode 22, was measured as film thickness of a coated film formed on an interior surface of the substrate As a result, the film formed on the exterior surface A of the substrate had a thickness of 20 um, and the film formed on the interior surface G had a thickness of 6 um. The Ra value of the cured coated film on the surface A was measured to be 0.21 um.

Comparative Example 2

[0121] A cured coated film was obtained according to substantially the same manner as described in Comparative example 1, except that a voltage of 260 V was applied between the steel plates and the electrode. The film formed on the exterior surface A of the substrate had a thickness of 30 um, and the film formed on the interior surface G had a thickness of 12 um. The Ra value of the cured coated film on the surface A was measured to be 0.19 um.

Comparative Example 3

[0122] A cured coated film was obtained according to substantially the same manner as described in Example 1, except that the first electrodeposition coating composition obtained in Preparation example 6 was employed instead of the second electrodeposition coating composition obtained in Preparation example 7 (That is, the electrodeposition coating step was conducted in twice with using the first electrodeposition coating composition obtained in Preparation example 6.), further except that the first coating step was conducted under 170 V for 3 minutes, and the second coating step was conducted under 230 V for 3 minutes.

[0123] The film formed on the exterior surface A of the substrate had a thickness of 17 um, and the film formed on the interior surface G had a thickness of 5 um. The Ra value of the cured coated film on the surface A was measured to be 0.12 um. 

What is claimed is:
 1. An electrodeposition coating process comprising: a first electrodeposition step in which an electrodeposition coated film is partially formed on a substrate surface to be coated with using a lead-free cationic electrodeposition coating composition which comprises an aqueous medium, a binder resin composed of a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in the aqueous medium, a neutralizing acid in order to neutralize the cationic epoxy resin, an organic solvent, and a metal catalyst, and which has a volatile organic content of 1% by weight or less, a metal ion content of 500 ppm or less and a neutralizing acid amount of 10 to 30 mg equivalent based on 100 g of binder resin solid content, and an electrodeposition coated film deposited on the substrate surface from the coating composition has a glass transition temperature of not more than 0° C.; a second electrodeposition step in which an electrodeposition coated film is formed on an uncoated portion of the substrate surface with using a lead-free cationic electrodeposition coating composition which comprises an aqueous medium, a binder resin composed of a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in the aqueous medium, a neutralizing acid in order to neutralize the cationic epoxy resin, an organic solvent, and a metal catalyst, and which has a volatile organic content of 1% by weight or less, a metal ion content of 500 ppm or less and a neutralizing acid amount of 10 to 30 mg equivalent based on 100 g of binder resin solid content, and an electrodeposition coated film deposited on the substrate surface from the coating composition has a glass transition temperature of 5 to 20° C.; and a step of baking the electrodeposition coated film to cure.
 2. The electrodeposition coating process according to claim 1, wherein the first electrodeposition step and the second electrodeposition step are conducted with using separate electrodeposition baths respectively.
 3. The electrodeposition coating process according to claim 1, wherein the electrodeposition coated film formed in the first electrodeposition step has a thickness of 8 to 20 um.
 4. The electrodeposition coating process according to claim 1, wherein the metal ion of the lead-free cationic electrodeposition coating composition is one or more selected from the group consisting of cerium ion, bismuth ion, copper ion, zinc ion, molybdenum ion, and aluminum ion.
 5. The electrodeposition coating process according to claim 1, wherein the neutralizing acid of the lead-free cationic electrodeposition coating composition is one or more selected from the group consisting of acetic acid, lactic acid, formic acid, and sulfamic acid.
 6. The electrodeposition coating process according to claim 1, wherein the lead-free cationic electrodeposition coating composition further comprises a pigment in a ratio of 1/9 or less by weight based on a resin solid contained in the coating composition. 